J Pole Antenna Voltage, Current and Radiation Pattern

J pole antenna is a vertical omnidirectional antenna popular among Ham Radio operators for 2m and 70 cm operations. As it is an end fed antenna, the feed point of the half wave segment of the antenna will have high impedance which has to be matched to the feedline by a shorted quarter wave parallel transmission line stub. The point of attachment of the feeder coax is determined by sliding the connection back and forth along the stub monitoring for the lowest SWR. J pole antenna has a gain of about 2dB compared to a quarter wave ground plane antenna. The gain is slightly more on the side of the J stub compared to the opposite side.


Current and voltage patterns in the J-pole antenna is illustrated in the diagram. Current peaks at lower end of the 3/4λ section while voltage peaks at the upper end. For the quarter wave stub current peak is at the lower end and voltage peak at the upper end.

The parallel conductors of the J pole antenna should be kept away from other conducting material by a distance of two to three times the spacing between the conductors. Best performance is obtained by having a non conducting free space between the antenna conductors and the mounting structure. Separation between the two conductors should not be more than 2% of the wavelength. An important advantage of the J pole is that it does not require radials or a ground plane. This reduces the wind resistance. A coil of coax at the feedpoint can serve as a choke to prevent radiofrequency from travelling down along the feedline.

Dual band operation is feasible for J-Pole antenna as it resonates on the third harmonic of fundamental frequency as well. Hence it may be useful in satellite communications as a VHF J-Pole antenna works also at UHF, being third harmonic in the usual amateur radio band. It is mentioned that the radiation pattern at third harmonic is not so favourable for terrestrial communication for J-Pole antenna. This diagram illustrates the approximate radiation pattern of a J-Pole antenna in the E-Plane.

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